http://en.wikipedia.org/wiki/Windows_NT_3.1
I was able to sign up and get windows 3.1 and windows NT back in the day, I forget if I was invited for win 3.1 but the wiki does show that they had public betas for NT
Just from my memory the Window 7 was by far the biggest and open Beta they did, and unique in the way it was implemented.
http://arstechnica.com/information-...ws-7-had-8-million-testers-biggest-beta-ever/
I wish they had it for windows 8, we could have got 8.1 features earlier which greatly improved on 8.